Thanks for the review. Committed to trunk after addressing the comments below.

1. The command is already there. We can file new jiras to make it an api.2. This is added since it's useful to know when a high watermark doesn't exist.3. This is because the usage of broker list is different btw 0.7 and 0.8. In 0.7, it's used to send the produce requests. In 0.8 it's only used to send metadata requests.